There is a large number of applications which require an imaging or mapping system. In a non exhaustive manner, the aerospace, meteorology, oceanography and astronomy applications can be quoted.
These systems must offer high performances under many aspects, which leads to requirements which, even if not contradictory, often are hard to conciliate.
It is often desirable to simultaneously obtain a large space resolution, a high sensitivity, a wide exploration zone, a short revisit time, and a continuous coverage, while keeping a reasonable degree of electric, but mostly mechanical, complexity. The underlying reason for the difficulty in obtaining these characteristics is that a radiometer can not synthesize a radiating aperture while using the orbital movement to observe the thermal radiation of the earth globe, which are comprised in a broad frequency range. This fact limits the space resolution obtainable by a radiometric system to the beam width of a radiating aperture which is no larger than the physical extension of its antenna.
A promising concept for obtaining the optimal possible performances in hyperfrequency radiometry, consists of combining a conical scanning and a "pushbroom" with the help of several beams.
Among the systems known in the art, the following, non limiting, examples, can be quoted: the "MIMR" or Multi-frequency Imaging Microwave Radiometer and the "EFAM" or Extreme Floods Alarm Mission.
The first project is described in the article of R. BORDI et al.: "MIMR Radiometer: Design, Calibration and expected Performance", "Microrad Conference", 1994.
The "MIMR" uses a combination of a conical scanning and a pushbroom in two high frequency channels, namely 36.5 and 89 GHz. The pushbroom beams are generated by means of two parallel antenna power supply sources, and the receiving systems share a single main reflector. The beam rotation results from a mechanical rotation movement of the instrument, in its entirety. In order to obtain a continuous coverage from an orbit of a particular altitude, the sensor needs to rotate at a predetermined speed.
More precisely, for a 50 degrees incident angle and when using a 1.5 m antenna radiating aperture, the ground space resolution respectively is 12 km and 4.5 km for the 36.5 Hz and 89 GHz channels. The instrument is on a 800 km orbit. Two beams are provided in the 36.5 GHz channel and four beams in the 89 GHz channel.
The second project is described in the article of Pierdicca and al.: "Observing Storm Clouds by spaceborne Multifrequency Microwave Radiometers", "ESA Earth Observation Quarterly", No 49, 1995. This example is particularly interesting since it concerns a radiometer application to meteorology and, more particularly, to the observation of clouds and rain precipitation. The interest resides in that the space resolution should be high and the revisit time should be short, for this application. These requirements are difficult to meet since this type of mission is based on small satellite configurations. The antenna size thus also is limited, 1 m at most, which results in a ground level space resolution of 10 km at 36.5 GHz and 20 km at 89 GHz, from a 600 km orbit. A 30 rpm rotation speed needs to be communicated. An exploration zone of 1000 km is reached for one single satellite. There follows that several satellites, typically four to six satellites, are required to obtain a short revisit time.
The present tendencies can be summarized as follows: in correlation with more stringent requirements for space resolution, an endless spiraling evolution towards a growing mechanical complexity is observed, for the reasons described hereafter.
Namely, the higher the space resolution is, the larger the antenna size, and the faster the radiometer rotation speed, must be in order to obtain a continuous coverage. There follows that the integration time available per pixel decreases, which in its turn degrades the sensitivity. The sensibility may of course be restored by providing additional "pushbroom" beams, which allows reverting to the original rotation speed. This beam addition however in its turn causes a larger antenna size due to the larger angular range required by the clearance of the beams, each of which illuminates an effectively different reflector zone. The finally obtained configuration implies a large size reflector, because of the increased space resolution and the added scanning beams. This configuration also includes a larger number of receivers, also due to the added beams, which rotate at the original speed. The feasibility of this mechanical configuration is very questionable since it is well known that the moment of inertia, and consequently the couple, of the instrument increases exponentially with the reflector size and linearly with its mass.